Zoom lens system and electronic imaging apparatus using

ABSTRACT

A zoom lens system includes a negative first lens group, a positive second lens group, and a negative third lens group, in that order from the object side, wherein upon zooming from the short focal length extremity to the long focal length extremity, the distance between the first lens group and the second lens group decreases. The third lens group includes a negative lens element having a concave surface on the image side, a positive lens element having a convex surface on the image side, and a negative lens element having convex surface on the image side, in that order from the object side. An electronic imaging apparatus using this zoom lens system is also disclosed.

BACKGROUND OF THE INVENTION

1. Field of the Invention

The present invention relates to a zoom lens system and an electronicimaging apparatus using the same.

2. Description of Related Art

In the digital camera market, while there is a tendency for furtherminiaturization and a higher zoom ratio, there also is an increaseddemand for a higher definition (higher picture quality). One way toobtain a higher definition is to, for example, use an image sensor(image pickup device) that has a large APS size. In an SLR camera, theimage sensor is large, so that a high definition image can be achieved,however, since the overall camera system is large and heavy, demands forfurther miniaturization cannot be met. In recent years, a so-called“mirrorless” SLR camera has been proposed in which the quick-returnmirror, which is a major characteristic feature of SLR cameras, isremoved and does not use an optical finder system; however, it cannot besaid that such mirrorless SLR cameras achieve sufficientminiaturization. Furthermore, lens-shutter zoom lens systems whichpursue miniaturization have also been proposed, however, theincidence-angle of light that is incident on the imaging surface is notsufficiently perpendicular to the imaging surface (i.e., has poortelecentricity), so that use of such lens-shutter zoom lens systems witha digital imaging sensor is not realistic.

Conventionally, a zoom lens system is known in the art which has threelens groups, i.e., a negative first lens group, a positive second lensgroup, and a negative third lens group, in that order from the objectside, for use with an image sensor having a large APS size. In thisconventional zoom lens system, it is typical for the third lens group tobe configured of two lens elements, i.e., a negative lens element and apositive lens element, in that order from the object side, or a positivelens element and a negative lens element, in that order from the objectside. Examples of the latter arrangement of the third lens group can befound in Japanese Unexamined Patent Publication Nos. 2001-290076,2002-221660 and 2009-25534.

However, although the former zoom lens system in which the third lensgroup is configured of a negative lens element and a positive lenselement, in that order from the object side, is advantageous forimproving telecentricity, various aberrations such as lateral chromaticaberration, astigmatism, distortion and spherical aberration that occurin the negative first lens group cannot be favorable corrected.Furthermore, since the negative lens element on the object side withinthe third lens group and the positive lens element on the image sidewithin the third lens group constitute a local retrofocus lensarrangement, an increase in the overall length of the zoom lens systemcannot be avoided.

Whereas, in the latter zoom lens system in which the third lens group isconfigured of a positive lens element and a negative lens element, inthat order from the object side, in order to secure the negativerefractive power of the third lens group, the refractive power of thenegative lens element on the image side needs to be increased, so thatthe refractive-power balance between the positive lens element on theobject side and the negative lens element on the image side is notfavorable.

SUMMARY OF THE INVENTION

In view of the above-described problems, the present invention providesa zoom lens system configured of three lens groups, i.e., a negativelens group, a positive lens group and a negative lens group, in thatorder from the object side, which uses, e.g., a large APS-sized imagesensor while achieving compact zoom lens system that has a superioroptical quality; furthermore, the present invention also provides anelectronic imaging apparatus which utilizes such a zoom lens system.

According to an aspect of the present invention, a zoom lens system isprovided, including a negative first lens group, a positive second lensgroup, and a negative third lens group, in that order from the objectside, wherein upon zooming from the short focal length extremity to thelong focal length extremity, the distance between the first lens groupand the second lens group decreases. The third lens group includes anegative lens element having a concave surface on the image side, apositive lens element having a convex surface on the image side, and anegative lens element having convex surface on the image side, in thatorder from the object side.

It is desirable for the following condition (1) to be satisfied:

0.8<f3a/f3c<3.0  (1),

wherein f3a designates the focal length of the negative lens elementwhich is provided closest to the object side within the third lensgroup, and f3c designates the focal length of the negative lens elementwhich is provided closest to the image side within the third lens group.

It is desirable for the third lens group to include a focusing lensgroup which is moved along the optical axis direction during focusing,wherein the following condition (2) is satisfied:

0.65<Z3/Z<0.80  (2),

wherein

Z3=m3t/m3w,

Z=ft/fw,

m3t designates the lateral magnification of the third lens group at thelong focal length extremity when focused on an object at infinity, m3wdesignates the lateral magnification of the third lens group at theshort focal length extremity when focused on an object at infinity, ftdesignates the focal length of the entire the zoom lens system at thelong focal length extremity, and fw designates the focal length of theentire the zoom lens system at the short focal length extremity.

It is desirable for the following conditions (3) and (4) to besatisfied:

1.0<f1/f3<2.5  (3), and

−0.90<f2/f3<−0.60  (4),

wherein f1 designates the focal length of the first lens group, f2designates the focal length of the second lens group, and f3 designatesthe focal length of the third lens group.

In an embodiment, an electronic imaging apparatus is provided, includingthe above-described zoom lens system, and an image sensor which convertsan image formed by the zoom lens system into electrical signals.

According to the present invention, a zoom lens system configured ofthree lens groups, i.e., a negative lens group, a positive lens groupand a negative lens group, in that order from the object side, isachieved which uses, e.g., a large APS-sized image sensor whileachieving compact zoom lens system that has a superior optical quality;furthermore, the present invention also provides an electronic imagingapparatus which utilizes such a zoom lens system.

The present disclosure relates to subject matter contained in JapanesePatent Application No. 2010-256911 (filed on Nov. 17, 2010) which isexpressly incorporated herein in its entirety.

BRIEF DESCRIPTION OF THE DRAWINGS

The present invention will be discussed below in detail with referenceto the accompanying drawings, in which:

FIG. 1 shows a lens arrangement of a first numerical embodiment of azoom lens system, according to the present invention, at the long focallength extremity when focused on an object at infinity;

FIGS. 2A, 2B, 2C and 2D show various aberrations that occurred in thelens arrangement shown in FIG. 1;

FIG. 3 shows a lens arrangement of the first numerical embodiment of thezoom lens system, according to the present invention, at the short focallength extremity when focused on an object at infinity;

FIGS. 4A, 4B, 4C and 4D show various aberrations that occurred in thelens arrangement shown in FIG. 3;

FIG. 5 shows a lens arrangement of a second numerical embodiment of azoom lens system, according to the present invention, at the long focallength extremity when focused on an object at infinity;

FIGS. 6A, 6B, 6C and 6D show various aberrations that occurred in thelens arrangement shown in FIG. 5;

FIG. 7 shows a lens arrangement of the second numerical embodiment ofthe zoom lens system, according to the present invention, at the shortfocal length extremity when focused on an object at infinity;

FIGS. 8A, 8B, 8C and 8D show various aberrations that occurred in thelens arrangement shown in FIG. 7;

FIG. 9 shows a lens arrangement of a third numerical embodiment of azoom lens system, according to the present invention, at the long focallength extremity when focused on an object at infinity;

FIGS. 10A, 10B, 10C and 10D show various aberrations that occurred inthe lens arrangement shown in FIG. 9;

FIG. 11 shows a lens arrangement of the third numerical embodiment ofthe zoom lens system, according to the present invention, at the shortfocal length extremity when focused on an object at infinity;

FIGS. 12A, 12B, 12C and 12D show various aberrations that occurred inthe lens arrangement shown in FIG. 11; and

FIG. 13 shows a zoom path of the zoom lens system according to thepresent invention.

DESCRIPTION OF THE EMBODIMENTS

The zoom lens system according to the present invention, as shown in thezoom path of FIG. 13, is configured of a negative first lens group G1, apositive second lens group G2 and a negative third lens group G3, inthat order from the object side. A diaphragm S which is provided inbetween the first lens group G1 and the second lens group G2 movesintegrally with the second lens group G2 during zooming. ‘I’ designatesthe imaging plane. The third lens group G3 constitutes a focusing lensgroup which is moved during a focusing operation (the third lens groupG3 is advanced toward the image side upon carrying out a focusingoperation on an object at infinity to an object at a finite distance).

The zoom lens system, upon zooming from the short focal length extremity(WIDE) to the long focal length extremity (TELE), moves the firstthrough third lens groups G1 through G3 in the optical axis directionwhile reducing the distance between the first and second lens groups G1and G2, and reducing the distance between the second and third lensgroups G2 and G3.

More specifically, as shown in the zoom path of FIG. 13, in each of thefirst through third numerical embodiments, upon zooming from the shortfocal length extremity to the long focal length extremity, the firstthrough third lens groups G1 through G3 each move monotonically towardthe object side.

In each of the first through third numerical embodiments, the first lensgroup G1 is configured of a negative lens element 11 and a positive lenselement 12, in that order from the object side.

In each of the first through third numerical embodiments, the secondlens group G2 is configured of a cemented lens formed from a positivelens element 21 and a negative lens element 22; and a positive lenselement 23, in that order from the object side. The positive lenselement 23 has an aspherical surface on each side thereof.

In each of the first through third numerical embodiments, the third lensgroup G3 is configured of a negative lens element (a negative lenselement having a concave surface on the image side) 31, a positive lenselement (a positive lens element having a convex surface on the imageside) 32, and a negative lens element (a negative lens element having aconvex surface on the image side) 33, in that order from the objectside. In each of the first through third numerical embodiments, thenegative lens element 31 has an aspherical surface on the object sidethereof. In the first and second numerical embodiments, the positivelens element 32 has an aspherical surface on both sides thereof, whereasin the third numerical embodiment, the positive lens element 32 has anaspherical surface only on the image side thereof.

The zoom lens system of the present invention constitutes a retrofocuslens system at the short focal length extremity having a negative frontlens group (first lens group G1) and a positive rear lens group (secondand third lens groups G2 and G3), thereby improving telecentricity ofthe zoom lens system; and constitutes a telephoto lens system at thelong focal length extremity having a positive front lens group (firstand second lens groups G1 and G2) and a negative rear lens group (thirdlens group G3), thereby reducing the overall length of the zoom lenssystem. According to this lens arrangement, both an improvement in thetelecentricity and a reduced overall length of the zoom lens system canboth be achieved.

In order to improve the telecentricity of the zoom lens system, it isappropriate to configure the negative third lens group G3 of a negativelens element and a positive lens element, in that order from the objectside. However, if the third lens group G3 is only configured of two lenselements, i.e., a negative lens element and a positive lens element, inthat order from the object side, the various aberrations that occur inthe first lens group G1 such as lateral chromatic aberration,astigmatism, distortion and spherical aberration, etc., cannot befavorably corrected.

Therefore, in the zoom lens system according to the present invention,by providing a negative lens element (33) closest to the image sidewithin the third lens group G3, and configuring the third lens group G3so as to have three lens elements, i.e., a negative lens element, apositive lens element and a negative lens element, in that order fromthe object side, the telecentricity can be improved while the variousremaining aberrations that occurred at the first lens group G1 can befavorably corrected by the third lens group G3. Furthermore, the effectof such an arrangement is most apparent when the third lens group G3 isconfigured of the following three lens elements: a negative lens elementhaving a concave surface on the image side, a positive lens elementhaving a convex surface on the image side, and a negative lens elementhaving a convex surface on the image side, in that order from the objectside, as in the numerical embodiments of the zoom lens system.

Condition (1) specifies the ratio of the focal length of the negativelens element 31 provided closest to the object side within the thirdlens group G3 to the focal length of the negative lens element 33provided closest to the image side within the third lens group G3, andis for improving the telecentricity of the zoom lens system while alsocorrecting the various aberrations that occurred at the first lens groupG1.

If the upper limit of condition (1) is exceeded, the refractive power ofthe negative lens element 33 that is provided closest to the image sidewithin the third lens group G3 becomes too strong, which is advantageousfor correcting various aberrations that occurred at the first lens groupG1, however, it becomes difficult to maintain telecentricity.

If the lower limit of condition (1) is exceeded, the refractive power ofthe negative lens element 31 that is provided closest to the object sidewithin the third lens group G3 becomes too strong, which is advantageousfor improving telecentricity, however, it becomes difficult to correctthe various aberrations that occurred at the first lens group G1.

As described above, the third lens group G3 constitutes a focusing lensgroup which is moved along the optical axis during a focusing operation.

With respect to the above-described lens arrangement, condition (2)specifies the proportion of the ratio of the lateral magnification ofthe third lens group G3 at the long focal length extremity when focusedon an object at infinity to the lateral magnification of the third lensgroup G3 at the short focal length extremity when focused on an objectat infinity (i.e., the zoom ratio of the third lens group G3), to theratio of the focal length of the entire zoom lens system, at the longfocal length extremity to the focal length of the entire zoom lenssystem at the short focal length extremity. By satisfying condition (2),a focal shift that would otherwise occur during zooming can beprevented. Namely, by satisfying condition (2), the change in the zoomratio of the entire zoom lens system can be made to coincide with thechange of focusing sensitivity of the third lens group G3, whichconstitutes a focusing lens group, so that a focal shift which wouldotherwise occur during zooming can be prevented.

If either the upper or lower limits of condition (2) is exceeded, thefocal shift during zooming increases.

Condition (3) specifies the ratio of the focal length of the first lensgroup G1 to the focal length of the third lens group G3, and is forachieving both an improvement of telecentricity and furtherminiaturization of the zoom lens system.

If the upper limit of condition (3) is exceeded, the refractive power ofthe third lens group G3 becomes too strong, which is advantageous forminiaturization, however, the telecentricity undesirably deteriorates.

If the lower limit of condition (3) is exceeded, the refractive power ofthe third lens group G3 becomes too weak, so that sufficientminiaturization cannot be achieved.

Condition (4) specifies the ratio of the focal length of the second lensgroup G2 to the focal length of the third lens group G3, and is for bothcorrecting aberrations and improving telecentricity.

If the upper limit of condition (4) is exceeded, the refractive power ofthe second lens group G2 becomes too strong, so that it becomesdifficult to correct spherical aberration and coma, especially at thelong focal length extremity.

If the lower limit of condition (4) is exceeded, the refractive power ofthe third lens group G3 becomes too strong, so that the telecentricityundesirably deteriorates.

Specific numerical embodiments will be herein discussed. The followingnumerical embodiments are applied to a zoom lens system used in adigital camera. In the aberration diagrams and the tables, the d-line,the g-line and the C-line show aberrations at their respectivewave-lengths; S designates the sagittal image, M designates themeridional image, FNO. designates the f-number, f designates the focallength of the entire optical system, W designates the half angle of view(°), Y designates the image height, fB designates the backfocus, Ldesignates the overall length of the lens system, r designates theradius of curvature, d designates the lens thickness or distance betweenlenses, N(d) designates the refractive index at the d-line, and vddesignates the Abbe number with respect to the d-line. The values forthe f-number, the focal length, the half angle-of-view, the imageheight, the backfocus, the overall length of the lens system, and thedistance between lenses (which changes during zooming) are shown in thefollowing order: short focal length extremity, intermediate focallength, and long focal length extremity.

An aspherical surface which is rotationally symmetrical about theoptical axis is defined as:

x=cy ²/(1+[1−{1+K}c ² y ²]^(1/2))+A4y ⁴ +A6y ⁶ +A8y ⁸ +A10y ¹⁰ +A12y ¹². . . .

wherein ‘x’ designates a distance from a tangent plane of the asphericalvertex, ‘c’ designates the curvature (1/r) of the aspherical vertex, ‘y’designates the distance from the optical axis, ‘K’ designates the coniccoefficient, A4 designates a fourth-order aspherical coefficient, A6designates a sixth-order aspherical coefficient, A8 designates aneighth-order aspherical coefficient, A10 designates a tenth-orderaspherical coefficient, and A12 designates a twelfth-order asphericalcoefficient.

Numerical Embodiment 1

FIGS. 1 through 4D and Tables 1 through 4 show a first numericalembodiment of a zoom lens system according to the present invention.FIG. 1 shows a lens arrangement of the first numerical embodiment of thezoom lens system at the long focal length extremity when focused on anobject at infinity. FIGS. 2A, 2B, 2C and 2D show various aberrationsthat occurred in the lens arrangement shown in FIG. 1. FIG. 3 shows alens arrangement of the first numerical embodiment of the zoom lenssystem at the short focal length extremity when focussed on an object atinfinity. FIGS. 4A, 4B, 4C and 4D show various aberrations that occurredin the lens arrangement shown in FIG. 3. Table 1 shows the lens surfacedata, Table 2 shows various zoom lens system data, Table 3 shows theaspherical surface data, and Table 4 shows the lens group data of thezoom lens system according to the first numerical embodiment.

The zoom lens system of the first numerical embodiment is configured ofa negative first lens group G1, a positive second lens group G2 and anegative third lens group G3, in that order from the object side. Thethird lens group G3 constitutes a focusing lens group that is movedalong the optical axis direction during a focusing operation (the thirdlens group G3 advances toward the image side when performing a focusingoperation while focusing on an object at infinity to an object at afinite distance).

The first lens group G1 (surface Nos. 1 through 4) is configured of abiconcave negative lens element 11 and a positive meniscus lens element12 having a convex surface on the object side, in that order from theobject side.

The second lens group G2 (surface Nos. 6 through 10) is configured of acemented lens formed from a biconvex positive lens element 21 and anegative meniscus lens element 22 having a convex surface on the imageside; and a biconvex positive lens element 23, in that order from theobject side. The biconvex positive lens element 23 has an asphericalsurface on each side thereof. A diaphragm S (surface No. 5), which isprovided in between the first lens group G1 and the second lens groupG2, integrally moves with the second lens group G2 along the opticalaxis during zooming.

The third lens group G3 (surface Nos. 11 through 16) is configured of abiconcave negative lens element (a negative lens element having aconcave surface on the image side) 31, a positive meniscus lens element(a positive lens element having a convex surface on the image side) 32having a convex surface on the image side, and a negative meniscus lenselement (a negative lens element having a convex surface on the imageside) 33 having a convex surface on the image side, in that order fromthe object side. The biconcave negative lens element 31 has anaspherical surface on the object side. The positive meniscus lenselement 32 has an aspherical surface on each side thereof. An opticalfilter OP (surface Nos. 17 and 18) is disposed behind (between the thirdlens group G3 and the imaging plane I) the third lens group G3 (thenegative meniscus lens element 33).

TABLE 1 SURFACE DATA Surf. No. r d Nd νd  1 −1334.671 1.200 1.61800 63.4 2 12.871 3.300  3 16.395 2.200 1.84666 23.8  4 25.167 d4  5(Diaphragm)∞ 1.000  6 14.140 4.215 1.49700 81.6  7 −15.047 1.100 1.90366 31.3  8−75.813 2.834  9* 29.029 3.823 1.59201 67.0 10* −17.340 d10 11* −104.0291.500 1.52538 56.3 12 25.683 1.215 13* −54.738 2.360 1.63548 23.9 14*−28.914 4.225 15 −8.870 1.200 1.58913 61.2 16 −19.156 d16 17 ∞ 2.0001.51633 64.1 18 ∞ — The asterisk (*) designates an aspherical surfacewhich is rotationally symmetrical with respect to the optical axis.

TABLE 2 ZOOM LENS SYSTEM DATA Zoom Ratio 2.85 Short Long Focal LengthIntermediate Focal Length Extremity Focal Length Extremity FNO. 3.6 4.96.7 f 18.60 28.01 53.00 W 43.0 28.2 15.1 Y 14.24 14.24 14.24 fB 3.003.00 3.00 L 59.72 61.65 73.30 d4 16.072 10.253 2.000 d10 5.471 3.1781.354 d16 3.003 13.049 34.774

TABLE 3 Aspherical Surface Data (the aspherical surface coefficients notindicated are zero (0.00)): Surf. No. K A4 A6 A8 9 0.000 −0.3003E−04  0.5680E−06 0.6042E−08 10 0.000 0.1112E−03 0.3802E−06 0.8883E−08 11 0.0000.5288E−04 −0.4674E−07   13 0.000 0.1021E−03 0.2720E−05 −0.6838E−08   140.000 0.5377E−04 0.1964E−05 0.6335E−08

TABLE 4 LENS GROUP DATA Lens Group 1^(st) Surf. Focal Length 1 1 −36.832 6 15.68 3 11 −19.04

Numerical Embodiment 2

FIGS. 5 through 8D and Tables 5 through 8 show a second numericalembodiment of a zoom lens system according to the present invention.FIG. 5 shows a lens arrangement of the second numerical embodiment ofthe zoom lens system at the long focal length extremity when focused onan object at infinity. FIGS. 6A, 6B, 6C and 6D show various aberrationsthat occurred in the lens arrangement shown in FIG. 5. FIG. 7 shows alens arrangement of the second numerical embodiment of the zoom lenssystem at the short focal length extremity when focussed on an object atinfinity. FIGS. 8A, 8B, 8C and 8D show various aberrations that occurredin the lens arrangement shown in FIG. 7. Table 5 shows the lens surfacedata, Table 6 shows various zoom lens system data, Table 7 shows theaspherical surface data, and Table 8 shows the lens group data of thezoom lens system according to the second numerical embodiment.

The lens arrangement of the second numerical embodiment is the same asthat of the first numerical embodiment except for the following points:

-   (1) The positive lens element 23 of the second lens group G2 is a    positive meniscus lens element having a convex surface on the image    side.-   (2) The negative lens element 31 of the third lens group G3 is a    negative meniscus lens element having a convex surface on the object    side.

TABLE 5 SURFACE DATA Surf. No. r d Nd νd  1 −617.730 1.200 1.61800 63.4 2 13.287 3.300  3 16.041 2.200 1.84666 23.8  4 23.384 d4  5(Diaphragm)∞ 1.000  6 13.168 4.600 1.49700 81.6  7 −13.157 1.100 1.90366 31.3  8−42.053 1.771  9* −113.546 4.460 1.59201 67.0 10* −12.046 d10 11*176.416 1.500 1.52538 56.3 12 20.741 1.610 13* −79.945 2.360 1.6064127.2 14* −29.955 3.830 15 −8.993 1.200 1.58913 61.2 16 −21.985 d16 17 ∞2.000 1.51633 64.1 18 ∞ — The asterisk (*) designates an asphericalsurface which is rotationally symmetrical with respect to the opticalaxis.

TABLE 6 ZOOM LENS SYSTEM DATA Zoom Ratio 2.85 Short Long Focal LengthIntermediate Focal Length Extremity Focal Length Extremity FNO. 3.6 4.86.7 f 18.60 28.01 53.00 W 43.0 28.3 15.1 Y 14.24 14.24 14.24 fB 4.004.00 4.00 L 60.13 61.46 73.25 d4 16.070 9.971 2.000 d10 5.746 3.4011.394 d16 2.178 11.960 33.728

TABLE 7 Aspherical Surface Data (the aspherical surface coefficients notindicated are zero (0.00)): Surf. No. K A4 A6 A8 9 0.000 −0.1929E−03  −0.9473E−06   10 0.000 0.1392E−04 −0.3276E−06   11 0.000 0.2739E−040.1642E−06 13 0.000 0.4397E−04 0.1392E−05 0.6448E−08 14 0.000−0.2479E−04   0.7396E−06 0.1127E−07

TABLE 8 LENS GROUP DATA Lens Group 1^(st) Surf. Focal Length 1 1 −36.012 6 16.09 3 11 −20.16

Numerical Embodiment 3

FIGS. 9 through 12D and Tables 9 through 12 show a third numericalembodiment of a zoom lens system according to the present invention.FIG. 9 shows a lens arrangement of the third numerical embodiment of thezoom lens system at the long focal length extremity when focused on anobject at infinity. FIGS. 10A, 10B, 10C and 10D show various aberrationsthat occurred in the lens arrangement shown in FIG. 9. FIG. 11 shows alens arrangement of the third numerical embodiment of the zoom lenssystem at the short focal length extremity when focused on an object atinfinity. FIGS. 12A, 12B, 12C and 12D show various aberrations thatoccurred in the lens arrangement shown in FIG. 11. Table 9 shows thelens surface data, Table 10 shows various zoom lens system data, Table11 shows the aspherical surface data, and Table 12 shows the lens groupdata of the zoom lens system according to the third numericalembodiment.

The lens arrangement of the third numerical embodiment is the same asthat of the first numerical embodiment except for the following points:

-   -   (1) The negative lens element 11 of the first lens group G1 is a        negative meniscus lens element having a convex surface on the        object side.    -   (2) The negative lens element 31 of the third lens group G3 is a        negative meniscus lens element having a convex surface on the        object side.    -   (3) The positive meniscus lens element 32 of the third lens        group G3 has an aspherical surface only on the image side.

TABLE 9 SURFACE DATA Surf. No. r d Nd νd  1 436.154 1.200 1.61800 63.4 2 12.090 3.223  3 14.679 2.275 1.84666 23.8  4 21.358 d4  5(Diaphragm)∞ 2.550  6 15.119 5.946 1.49700 81.6  7 −12.709 3.801 1.90366 31.3  8−42.498 1.130  9* 58.643 3.725 1.59201 67.0 10* −16.736 d10 11* 35.8151.500 1.52538 56.3 12 15.480 3.177 13 −37.301 2.498 1.60641 27.2 14*−20.712 2.579 15 −10.047 1.200 1.60311 60.7 16 −26.977 d16 17 ∞ 2.0001.51633 64.1 18 ∞ — The asterisk (*) designates an aspherical surfacewhich is rotationally symmetrical with respect to the optical axis.

TABLE 10 ZOOM LENS SYSTEM DATA Zoom Ratio 2.85 Short Long Focal LengthIntermediate Focal Length Extremity Focal Length Extremity FNO. 3.6 4.96.7 f 18.60 28.00 52.99 W 43.0 28.5 15.2 Y 14.24 14.24 14.24 fB 1.001.00 1.00 L 62.41 65.34 81.45 d4 13.529 8.613 2.000 d10 7.464 4.0951.000 d16 3.617 14.826 40.651

TABLE 11 Aspherical Surface Data (the aspherical surface coefficientsnot indicated are zero (0.00)): Surf. No. K A4 A6 A8 9 0.000−0.6450E−04   0.3340E−06 10 0.000 0.3484E−04 0.3835E−06 11 0.0000.2090E−04 0.3044E−06 14 0.000 −0.5136E−04   −0.2850E−06   −0.3297E−09

TABLE 12 LENS GROUP DATA Lens Group 1^(st) Surf. Focal Length 1 1 −35.842 6 17.07 3 11 −22.63

The numerical values of each condition for each embodiment are shown inTable 13.

TABLE 13 Embod. 1 Embod. 2 Embod. 3 Cond. (1) 1.33 1.68 1.95 Cond. (2)0.72 0.71 0.75 Cond. (3) 1.93 1.79 1.58 Cond. (4) −0.82 −0.80 −0.75

As can be understood from Table 13, the first through third numericalembodiments satisfy conditions (1) through (4). Furthermore, as can beunderstood from the aberration diagrams, the various aberrations aresuitably corrected.

Obvious changes may be made in the specific embodiments of the presentinvention described herein, such modifications being within the spiritand scope of the invention claimed. It is indicated that all mattercontained herein is illustrative and does not limit the scope of thepresent invention.

1. A zoom lens system comprising a negative first lens group, a positivesecond lens group, and a negative third lens group, in that order fromthe object side, wherein upon zooming from the short focal lengthextremity to the long focal length extremity, the distance between saidfirst lens group and said second lens group decreases, wherein saidthird lens group includes a negative lens element having a concavesurface on the image side, a positive lens element having a convexsurface on the image side, and a negative lens element having convexsurface on the image side, in that order from the object side.
 2. Thezoom lens system according to claim 1, wherein the following condition(1) is satisfied:0.8<f3a/f3c<3.0  (1), wherein f3a designates the focal length of thenegative lens element which is provided closest to the object sidewithin said third lens group, and f3c designates the focal length of thenegative lens element which is provided closest to the image side withinsaid third lens group.
 3. The zoom lens system according to claim 1,wherein said third lens group comprises a focusing lens group which ismoved along the optical axis direction during focusing, wherein thefollowing condition (2) is satisfied:0.65<Z3/Z<0.80  (2),whereinZ3=m3t/m3w,Z=ft/fw, m3t designates the lateral magnification of said third lensgroup at the long focal length extremity when focused on an object atinfinity, m3w designates the lateral magnification of said third lensgroup at the short focal length extremity when focused on an object atinfinity, ft designates the focal length of the entire said zoom lenssystem at the long focal length extremity, and fw designates the focallength of the entire said zoom lens system at the short focal lengthextremity.
 4. The zoom lens system according to claim 1, wherein thefollowing conditions (3) and (4) are satisfied:1.0<f1/f3<2.5  (3), and−0.90<f2/f3<−0.60  (4), wherein f1 designates the focal length of thefirst lens group, f2 designates the focal length of the second lensgroup, and f3 designates the focal length of the third lens group.
 5. Anelectronic imaging apparatus comprising the zoom lens system accordingto claim 1, and an image sensor which converts an image formed by saidzoom lens system into electrical signals.